MOULDING, CASTING, AND PRESSING (Pottery, for artists)
CHAPTER IV MOULDING, CASTING, AND PRESSING "Our soundes is good, Our shapes is neat, Its Davis cast us so compleat." From an old bell at Stoke Rivers. THE casting process, employed so extensively in commercial work, is in its essence mechanical and therefore can never have the spontaneity or char- acter of thrown work. To-day when the thrower r;* MANDREL, ROlLIDlNPAPnU- FIG. 6 and turner with hand and eye trained for good shapes is rare indeed, it is often the only method by which the student can obtain large shapes of high finish for painting or glazing in transparent colours. In the process the shape is first designed and 34 35 EDGE. 'ONETIECl'MOVLD IOK- JOWL- METHOD OF MLEASINC ORIGINAL FIG. 7 carefully drawn on paper, allowance be- ing made for the shrinkage of the fin- ished pot which may be as much as 1 in 6 with some kinds of slip. It should be refined in profile with no returns that may bind or hold in the mould when dry- ing, yet it is obvious that simple shapes that can be built or thrown by the beginner are not suitable for casting. The shape correctly drawn, a mandrel, a steel tapering to a point the butt wormed to screw on a lathe, and long enough to give some play at either end, is rolled in stout paper, gummed at the edges, as in Fig. 6. This is removed and allowed to set, thus giving a paper shell just fitting the man- drel. Trim this square at the base and stick it upright ATWO-HECE MOULD. FIG. 8 36 POURING IN PLA/TER. PIG. 9 37 by means of a piece of clay on a well-oiled portion of a table. Around this as a centre is fitted a roll of linoleum, oiled inside and secured with string, with all its joints caulked with clay. The inside diameter of this cylinder should be about \" larger than the greatest diameter of the drawn shape. Enough superfine plaster to fill this is now mixed. To do this shake plaster by handfuls into a bowl of water until it appears to refuse more, pour off any surplus water, and stir with a wooden ladle or the hand, avoiding air bubbles. A little practice in casting plaster bats will give the experience necessary for mixing plaster. When well mixed and a slight thickening is perceptible, it is poured very carefully into the cylinder, the paper shell being kept upright in the centre. (Fig. 9.) In about 20 minutes the surface of the plaster will feel warm. It is now set and the linoleum is removed. The plaster cylinder, when dry, is fitted on the mandrel, and this screwed to the lathe head. Two or three chisels are now required. (Fig. 10.) The rest on the lathe is clamped in a convenient position and a cutting chisel held as shown (Fig. 11), cutting edge up. The cylinder is revolved briskly as indicated by arrow, and the shape is roughly hollowed out. Proceed gradually until the shape begins to emerge, taking care not to cut too deeply. (Fig. 12.) Towards the finish use the calipers frequently to check the measurements. 38 TqRNBsIQTOOL. CHISEL POINTS At top and bottom about \" waste is left (Fig. 13 A), turned straight, the actual line of top and base being slightly grooved in the plaster. The form may be pretty accurately finished with the chisels and then nicely smoothed with sandpaper. For this process the rotary movement is reversed. FIG. 10 PO/ITION Of TOOL FIG. 11 The shape is now removed from the lathe and is ready for moulding. With a kick wheel having a removable head the mandrel can be screwed on and the block turned 39 PAKTLY TCJltfsltD JLOCK ON LATHI FIG. 12 down in an upright position. The difficulty of steadying the tool renders this method somewhat unsuitable for this process, where perfect sym- metry and a high finish are required. The plaster should not be so dense for turning on the wheel or the kicking is apt to become very arduous and this tends to dislocate the set of the spindle. One of the best of ways is to draw a quantity of shapes, cast their blocks, and turn them on a hired lathe, preferably run by power. The finished shape is now well lathered with part- ing or stopping. This is made by boiling 1 Ib. soft soap, J Ib. Russian tallow, and a small piece of soda. Another stopping is made by dissolving 1 Ib. soft soap in 1 qt. water and stirring in J pt. paraffin oil. Several coatings may be necessary to impart 40 a waxy surface. When no longer absorbent, it is dried and slightly polished with a bit of cotton waste. At the finish the form should be clean, smooth, glossy, and non-absorbent. With simple shapes, as in Figs. 7 and 8, a one-piece or a two-piece mould can be made, and here the bot- tom waste is not necessary, but with any return or foot a three-piece mould will be required. The waxed shape is now divided perpendicularly exactly in half, by a pencil line. (A, Fig. 13.) It is then laid on its side and bedded in clay up to the pencil lines, the clay being sloped slightly down from the marks. (F, Fig. 13.) Box in now with well-soaped boards tight against top and bottom but allowing about l" at sides. Wipe the shape over with waste dipped in olive oil but leave no surplus oil on the surface. Plaster well mixed as before is poured in until about 1 J" to 2" above the greatest projection of the shape, great care being taken to avoid or dislodge air bubbles. (B, Fig. 13.) When the plaster is set but still warm, the shape is removed and the side of the plaster that rested on the clay trimmed flat and several joggles or natches are made. (C, Fig. 13.) The shape is now replaced exactly as cast and the new surfaces treated with parting and the whole slightly filmed with oil as before. Great care must be used, for any oil on the actual surface of the mould spoils the suction of the 41 MOULDING V T HAL? MOULDING i" 1 HALT MOULDING 2/ 40 HAL? i MOULDING BA/E ^ AMFTHOD D FIG. 13 42 plaster at that spot. Box in and then cast just as before. This gives two halves with waste top and bottom. The shape is now placed on the lathe and the bot- tom waste turned off, the base of the shape being slightly hollowed. The creator having arrived so far successfully may now unbend and scratch his mark on this new surface before well waxing it. Dowels are cut in the waste of the two halves as shown, the fresh parts soaped, all fitted together and slightly oiled, then boxed in as in D and E, Fig. 13. Plaster about 2" thick at the thinnest part is poured on and the mould is complete. When set, the shape is removed and the three parts trimmed on all the outside edges. (Fig. 14.) The three pieces are assem- bled, firmly tied up to prevent warping, and thoroughly dried. If preferred, the mould can be made cylindrical instead of square. This will give a more even suc- tion to the slip and may be worth the extra trouble. For casting purposes a refractory clay containing a good percentage of China clay, maturing at about 03-01 but remaining perfectly white and porous, will be required. Slip for casting is made thus: Clay, picked or broken into small pieces, is thrown into a bucket of warm water well slubbed up by hand and passed through a fine sieve (No. 80) with the aid of a stout 43 THE THREE PART/ OFTHEMO^LD. FIG. 14 brush and thinned to the consistency of thick cream. This should be matured for some days, frequently stirred, and again sieved before using. For very small or fragile shapes, a finer sieve (120) is advisable. The mould, quite dry and clean, is now slightly moistened with a scrupulously clean sponge and water, the parts assembled, corded, and firmly wedged, leaving the top free as in Fig. 15. The slip, thoroughly stirred, is poured in very gently to avoid bubbles. With awkward moulds, a tube or funnel should be used to prevent splashing. If the mould be placed on a whirler and turned to and fro, it will prevent the heavier matter in the slip settling too quickly. 44 As the slip sinks in the mould, the subsidence being due to the ab- sorption of the water by the plaster, the mould should be con- tinually filled up. After a few minutes the mouth is scraped free to test the de- posit. When this is thick enough, vary- ing, of course, with the size of the shape, MOCILD READY TOR /LI P - tne sli P is poured out FIG. 15 into another bucket. An unorthodox but often useful trick for strengthening long necks is to slide a piece of glass over the mouth and reverse the mould for a minute or so. The neck full of slip thus allows a slight extra deposit on the part that most needs it when we come to finishing off the lip. Let the mould drain a little over the slip bucket and then reverse to dry FIG. 16 FBEHMGTOP 45 slowly. When the wet look has disappeared from the surface of the slip, scrape the top free and run a knife around to prevent sticking as the shape con- tracts. (Fig. 16.) In a few hours it will be dry enough to permit of the sides being eased off and the shape left to dry on the base. (Fig. 17.) When tough enough to handle with safety, the waste and cast lines are trimmed and finished off, any air bubbles or holes broken down and filled with clay scraped from the waste or base. If this finishing is left until the shape be dry, it is impossible to hide such defects. The greatest care must be exercised in hand- ling cast shapes, as they are I ./H APE Kf ADY exceptionally fragile. J f OKTRIMM1NC When quite dry, the whole form should be carefully gone over with a very fine sandpaper. A superfine surface should be imparted by rubbing with the hands. When using transparent glazes, as with under-glaze painting, it is essential that all scratches FIG. 17 46 be removed, and especially must all sharp edges be eliminated on neck or shoulder, for the glaze running away from these places imparts a hide- ously cheap look to what otherwise may be a fine shape. All these points having received attention, the date is scratched on the bottom of the shape and it is now ready to biscuit. Generally speaking, it will be found that slip the consistency of cream is right for casting, possibly thicker for big open shapes, and after the right pro- portion is settled it is as well to test what it weighs to the pint. As will be readily seen, this process, whilst open to many objections, lends itself to shapes that are re- fined and delicate and to those that have flutings or raised ornament. Such decorations, or the spouts of jugs, may be modelled in wax on the plaster shape before casting and appear in reverse on the mould. Designs may be scratched on the mould or shape and show as a delicate tracery beneath the glaze. All these things, however, add to the difficulty of casting and should be approached by degrees and with restraint. For in unskilled hands the process lends itself to soulless and mechanical repetition. PRESSING Moulds having moderately wide mouths enable the potter to press his shapes instead of cast them. 47 This method in expert hands is even quicker than casting and has the advantage of imparting a sturdier look to large shapes. Pressing is also resorted to for those shapes to which it would be difficult for the slip to obtain free access. For the ordinary three-piece mould the procedure would be thus : The clay, well wedged and quite ROLLING OUT CLAY FIG. 18 plastic, is rolled out as described in Jigger and Jolley work, to a suitable thickness. (Fig. 18.) Butter cloth or fine linen will do instead of leather to roll the clay on. The insides of the three parts of the mould are sponged and pieces of the thin rolled clay roughly cut to fit them. These pieces are now fitted and well applied to the three parts by dabbing with the damp sponge. A soft close-textured sponge, or a soft felt dabber, is best for this operation. When 48 closely setting, the edges are trimmed and given a slight bevel. The top is cut straight. Then the mould is assembled and firmly tied. Some of the waste clay is rolled into thin ropes. With the bev- elled edges slightly moistened, these ropes are firmly wedged into the two side junctions and round the base. Where the mouth is large enough for the in- sertion of the hand this is not a difficult operation. If it be narrow, the two halves of the mould may be tied up and the joints welded together before they are assembled on the base. A coil of clay can then be placed on the edge of the base just clear of the two sides which are now fitted over and tied up. Then a stick sponge is used to join up the base to the sides. (Fig. 19.) After a little while the shape is fit to be removed and is finished in the usual way. Nothing can rival large thrown shapes for vigour or variety, but unfortunately they are not always within reach of even the good craftsman. Then this method offers the least objectionable substitute for them and in clever hands is capable of many fine results. The following method is used to mould handles or simple applied ornament. Handles, feet, masks, etc., are usually pressed and stuck on the dry shape with slip. To mould them some skill is necessary if the press is to be quite accurate and free from twist or ugly seams. 49 STICK-SPONGE WELPING BAStTt>5ttMcS. FIG. 19 One way, when the handle or foot is symmetrical, is to cut the model exactly in half. This must be done when the model is tough enough to handle with- out bending or distorting it. One half is laid cut side down upon a sheet of glass, and surrounded at a convenient distance with clay walls. Plaster 50 HALF Of MoqLp HANDLE FIG. 20 is now poured on to form one half of the mould, and allowed to set. It is then removed and the smooth surface joggled and clay washed (brushed over with claywater). The other half is then very carefully ap- plied to the half still embedded in the mould, the walls built round and the other part of the mould cast. Then all is trimmed up and a groove run round the form as shown. (Fig. 20.) For pressing, the form is well filled with clay and the two halves of the mould strongly pressed together. Any surplus clay will squeeze into the groove and when tough enough to remove the whole is "fettled" and finished before drying and sticking up. With care and practice this method is possible: Build walls and pour in enough plaster to form one half of the mould. Before it stiffens, very carefully press in the handle or ornament just up to the half- way line and allow to set. Joggle, claywash, and cast the second half. Finally, when the object is of any size, clay walls may be used as described in the chapter on Figurines. Categoría:Pottery, for artists, craftsmen & teachers